In general, an engine rotating meter for measuring the engine speed of an automobile or a motorbike is structured to induce the electromagnetic wave generated in an engine ignition system so as to be detected, and to extract and count the pulse corresponding to the engine speed from the noise pulse contained in the detected induction electromagnetic wave (for example, see Patent Document 1).
The known engine rotating meter as described above includes an antenna 1, an input circuit 2, a waveform shaping circuit 3, and an arithmetic circuit 4 as shown in FIG. 5. The input circuit 2, the waveform shaping circuit 3 and the arithmetic circuit 4 constitute a measurement processing circuit P.
In the engine rotating meter, the antenna 1 induces the electromagnetic wave generated from the ignition system of the engine to be detected, and transmits the pulse corresponding to the ignition timing of the engine among those derived from the antenna 1 to the input circuit 2. The pulse wave transmitted to the input circuit 2 is shaped into a rectangular wave in the waveform shaping circuit 3. The arithmetic circuit 4 calculates the engine speed based on the thus shaped rectangular wave pulse, and the calculated engine speed is displayed on a display device 5.
Generally, the engine output varies depending on the usage in the industrial field, for example, an automobile, a motorbike, a grass cutter and the like, and accordingly, the output energy or the frequency of the electromagnetic wave generated from the ignition system of the engine varies depending on the type of the machine. A level of the input pulse corresponding to the engine speed input to the measurement processing circuit P of the engine rotating meter may also differ.
In the case where the electromagnetic wave generated in the ignition system of the engine is at a high level, and accordingly, the level of the pulse input to the input circuit 2 is high as shown in FIG. 6(a), the level of the pulse input to the waveform shaping circuit 3 also becomes high as shown in FIG. 6(b). In the aforementioned case, besides the pulses required to be counted, the unnecessary adjacent pulse generated at the ignition timing of the engine as noise may be waveform-shaped in the waveform shaping circuit 3 simultaneously.
FIG. 6(c) is a chart indicating the case where two pulses are incorrectly waveform-shaped at a single ignition timing of the engine. As the engine speed is obtained based on the count values of the two pulses in the arithmetic circuit 4, the calculated engine speed is inaccurate, thus deteriorating the reliability of the engine rotating meter. The aforementioned generally employed engine rotating meter may accidentally count the external electromagnetic wave noise at high frequency, which is not originated from the engine ignition.
The large capacitance measurement processing circuit is preliminarily provided in order to withstand abnormally high input level or high input frequency. This case inevitably causes the circuit structure to be complex and the production cost to be increased.
Meanwhile, upon measurement of the engine speed, the input sensitivity of the electromagnetic wave is adjusted by moving the antenna to be close to or away from the point from where the electromagnetic wave is generated while observing (monitoring) the engine speed displayed on the display device of the engine rotating meter so as to prevent the input pulse level from becoming excessively high.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-113522